This invention relates to a protective, initially colorless ink for use on a document to protect it against alteration, and more particularly to a leach-resistant chemically sensitive ink which develops color on contact with oxidizing agents and a document printed therewith.
Safety paper is commonly used for making checks, money orders, negotiable certificates, and other documents of value or indicating value and which require protection against alteration or forgery. The safety paper typically has printed thereon chemically reactive inks which are colorless, but which develop color and reveal words such as "VOID" to show that an attempt has been made to alter the document when contacted with common oxidizing agents such as ink eradicators.
Remick, U.S. Pat. No. 1,839,995, describes such a safety paper which has been treated with an ink containing a guanidine substance which reacts with ink eradicators to cause a conspicuous color change on the paper.
Smith, U.S. Pat. No. 1,911,774, also relates to a safety paper which has been treated with an ink containing a combination of diphenol and diphenyl guanidine which produces a noticeable color change upon reaction with either an acidic or basic ink eradicator.
Currently, diphenyl guanidine and certain thiozol derivatives are used as protective inks for safety paper because they are nontoxic and relatively inexpensive. However, a major shortcoming of current safety papers is that they may be soaked in water and the protective inks leached free within a relatively short period of time (i.e., 30 minutes or less), rendering the security feature of the ink virtually ineffective against experienced forgers.
Accordingly, a need still exists in the art for a chemically sensitive protective ink which develops color on contact with an oxidizing agent and is also leach resistant once printed onto a document substrate over an extended period of time.